The present invention relates to an A/D conversion apparatus capable of performing A/D conversion with a reduced number of bits to obtain desired reception performance even if a level of a received signal varies greatly as in the mobile communication.
When digital signals are transmitted, it has been proposed that various processings such as equalization, RAKE receiving, removal of interference, detection of synchronization and the like are performed in the receiver side in order to improve the transmission quality. Most of them are effected by processing of signals, while it is often premised that the received signal is linearly received in order to improve the performance. On the other hand, in order to process the signal, it is necessary to sample the received signal by an AID converter, while the number of bits of the A/D converter is directly related to a cost thereof and accordingly it is impossible to increase the number of bits of the A/D converter. On the contrary, when the number of bits is small, satisfactory receiving performance is not obtained because of variation of the level due to the fading.
Accordingly, heretofore, two kinds of A/D conversion apparatuses have been developed in order to cope with a large dynamic range of the received signal. One of them uses an AGC amplifier to control an amplification factor at the preceding stage of the A/D conversion to absorb the dynamic range. The other converts amplitude information into logarithmic information to be A/D converted and samples phase information separately to thereby reproduce an original received signal from a combination of amplitude and phase information. The two kinds of conventional A/D conversion apparatuses are now described.
FIG. 6 schematically illustrates a conventional A/D conversion apparatus using an AGC amplifier. In FIG. 6. numeral 1 denotes a received signal, which is supplied through an AGC amplifier 2 and an A/D converter 3 to a demodulator 4. The AGC amplifier 2 changes its amplification factor in accordance with a level of the received signal 1 so that a dynamic range thereof is not increased excessively at an input of the A/D converter 3. Accordingly, even when the number of bits of the A/D converter 3 is small, the satisfactory demodulation performance is obtained in the demodulator 4.
FIG. 7 schematically illustrates another conventional A/D conversion apparatus in which a received signal is converted into a logarithmic signal to be sampled. In FIG. 7, numeral 11 denotes a received signal, which is supplied through a limiter amplifier 12, a phase detector 13 and an amplitude detector 14 to a demodulator 15. The limiter amplifier 12 is an amplifier which shapes the received signal 11 into a signal having a fixed amplitude regardless of an amplitude of the received signal 11 and an output signal of the limiter amplifier 12 lacks amplitude information but holds phase information. Accordingly, the phase can be obtained in the phase detector 13. On the other hand, the limiter amplifier 12 separately outputs a substantially logarithmic signal of an amplitude thereof and the signal is sampled by the amplitude detector (which may be a usual A/D converter). Since the amplitude is converted into the logarithmic signal, desired accuracy is obtained by the A/D converter with a reduced number of bits even if the dynamic range of the received signal is wide. Finally, the demodulator 15 reproduces the received signal 11 from the sampled value by the phase detector 13 and the sampled value by the amplitude detector 14 accurately and performs demodulation by using the reproduced result to thereby attain satisfactory demodulation performance.
In the method using the AGC amplifier, a loop by an analog signal or digital control is known as control of the amplification factor. However, any control method cannot control a suddenly produced burst signal sufficiently and an optimum solution of a control method such as a detection method of a level and a time constant of a feedback signal is different depending on the fading pitch or a state of lines. Accordingly, there is a problem that it is difficult to attain a desired operation under all conditions and a number of analog circuits are required.
Further, in the method in which the received level is converted into the logarithmic level to be sampled, the phase information and the amplitude information are obtained, while a large amount of calculations are required to reproduce the original signal from the phase and amplitude information and there is a tendency that a time difference occurs between the phase information and the amplitude information. Particularly, when a symbol rate is large, deterioration tends to occur.